The present invention relates to a studless tire, more particularly to a tread structure capable of improving on-ice performance as well as on-snow performance.
There have been proposed a variety of studless tires designed for use on icy roads and snowy roads for example as disclosed in U.S. Pat. No. 7,137,424 (JP-2003-63211-A). In the case of a studless tire, in order to provide traction and braking power on the icy roads, a large number of sipes are formed on tread elements such as block and rib since the sipes absorb and eliminate a water film on the icy road surface and increase the friction with the road surface. In order to improve the on-ice performance of a studless tire, it is necessary to increase the ground contacting area to increase the frictional force on the icy roads. On the other hand, in order to improve on-snow performance, it is necessary to increase the grooved area to increase the shear strength of snow column (snow grip). Here, the snow column is the snow on the road surface trodden by the tire and entered into the tread groove like a column rising from the road surface. As just described, to improve the on-ice performance and to improve the on-snow performance are contradictory problems.